Selective Attention • 89Stroop Eff ect
stimuli. The fl anker stimulus is presented off to the
side. In the examples shown in ● Figure 4.9, the X
is the target and the N is the fl anker. Participants
are told to respond to the target stimulus. When
the X is the target (as in this example), participants
are told to press one key; when the N is the target,
they are to press a different key. Because the X and
N are associated with different responses, the N is
an incompatible fl anker when the X is the target.
In Figure 4.9a, the task of responding to the
X is easy (low-load condition) because it is easy to
see the X among the O’s. In Figure 4.9b, the task
of responding to the X is more diffi cult (high-load
condition) because it is harder to fi nd among the
other letters. The results of experiments for these
two conditions, shown in Figure 4.9c, indicate that
the incompatible fl anker causes a slower response
in the low-load condition (like the result for the
letter display in Figure 4.8b) but has no effect in
the high-load condition.
These results correspond to the two condi-
tions in Figures 4.7a and b. The low-load condi-
tion corresponds to Figure 4.7a. Because cognitive
resources are available, the incompatible fl ank-
ers intrude and cause slower responding. The
high-load condition corresponds to Figure 4.7b.
Because no cognitive resources are available, the incompatible fl ankers have no effect.
This means that when you are involved in a low-load task, such as driving on a familiar
road, you are able to process additional information; in fact, you might do so even if
you don’t intend to. In contrast, if you are involved in a high-load task, such as driving
in a construction zone, potentially distracting stimuli are more easily ignored.
The ability to ignore task-irrelevant stimuli is a function not only of the load of
the task you are trying to do, but also of how powerful the task-irrelevant stimu-
lus is. For example, while focusing on solving a math problem you may be able
to ignore a conversation in the hallway, but a loud siren, indicating fi re, would
probably attract your attention. An example of a situation in which task-irrelevant
stimuli are diffi cult to ignore is provided by the Stroop effect, described in the fol-
lowing demonstration.DEMONSTRATION The Stroop Eff ect
Look at ● Figure 4.10. Your task is to name, as quickly as possible, the color of ink used to print
each of the shapes. For example, starting in the upper left corner, and going across, you would
say, “red, blue,.. .” and so on. Time yourself (or a friend you have enlisted to do this task), and
determine how many seconds it takes to report the colors of all of the shapes. Then repeat the
same task for ● Figure 4.11, remembering that your task is to specify the color of the ink, not
the color name that is spelled out.If you found it harder to name the colors of the words than the colors of the shapes,
then you were experiencing the Stroop effect, which was fi rst described by J. R. Stroop
in 1935. This effect occurs because the names of the words cause a competing response
(just as in the incompatible condition in the fl anker compatibility task) and therefore
slow responding to the target—the color of the ink. In the Stroop effect the task-
irrelevant stimuli are extremely powerful, because reading words is highly practiced and
has become so automatic that it is diffi cult not to read them (Stroop, 1935).● FIGURE 4.9 Stimuli for a fl anker compatibility task in which the load
is increased by adding additional stimuli to the display. In this example,
X is the target and N is the fl anker. The response to the fl anker, N, is
incompatible with the response to the target, X, in both (a) the easy task
(low-load) condition and (b) the hard task (high-load) condition.
(c) Results of this experiment indicate that the incompatible fl anker N
slowed responding in the low-load condition compared to responding
when the fl anker was compatible, but did not slow responding in the
high-load condition (Lavie, 2005).
Low
LoadLow-load
conditionXOTargetNOOOOKWNMZHXHigh-load
conditionHigh40Response time compared tocompatible flankers (ms)(a) (b) (c)
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